Fiber-optic networks are widely used to carry telecommunication transmissions. A simple fiber-optic network consists of a signal source, typically a laser, a network of fiber-optic cables to route the signal, and a receiver at the end of each branch. The receivers in fiber-optic networks are photodiodes and will only work properly for a certain range of optical signal power. Since power levels will vary in different branches of a network, the power must be controlled to keep the power in a particular branch in the operating range of the receivers. This can be done using variable attenuators.
Variable attenuators often function by blocking a portion of the optic signal. FIG. 1 shows a schematic of such a variable attenuator 100. An optical signal is carried by a fiber 101 embedded concentrically in a glass ferrule 102. The glass ferrule is attached to a Graded Index (GRIN) lens 103 which collimates the light and projects it in a beam 104. The attenuating element 105 blocks a fraction of the light, while the remainder is focused by a second GRIN lens 106 into another fiber 108 mounted concentrically in a glass ferrule 107.
The attenuation element 105 may typically have two different designs. The first type of attenuating element used was simply a solid shutter that was inserted into the beam 104. However, with such a device, the degree of attenuation is a non-linear function of the distance the element is inserted into the light beam. This makes it difficult to control the amount of attenuation. To improve upon this situation, a second type of shutter is used. In this case, the shutter element may be made from a transparent material coated with a layer of material of variable thickness, where the intensity of light transmitted through the layer of material varies with its thickness. By adjusting the profile of the thickness of the opaque material, the attenuation of the signal would vary linearly with the insertion displacement of the shutter element.
Even though the second type of conventional shutter is an improvement over the first type, it may be difficult to control the variable thickness of the coating so that the shutter has a light transmission that varies accurately with displacement. It may also be costly to fabricate a shutter element with a coating of variable thickness where the thickness of the coating must be controlled accurately. It is, therefore, desirable to provide an improved variable attenuator design which is less costly and easier to manufacture.
This invention is based on the observation that the process for fabricating the shutter element can be much simplified by forming a layer of opaque material on a transparent substrate, where the opaque layer forms a pattern so that the radiation transmission function of the opaque layer varies along a given direction, which is along a dimension of the shutter element. Preferably, the radiation transmission function of the opaque layer varies as a smooth linear function along the length of the shutter element. Then, by controlling the distance by which the shutter element is inserted into the path of the beam of radiation along the given direction, the amount of attenuation can be accurately controlled.
It is much easier to control the pattern formed by the opaque layer on the transparent substrate instead of controlling the thickness of a layer of material on the substrate. The cost of fabrication of the attenuator is, therefore, much reduced.
In a preferred embodiment, a large number of shutter elements may be formed at the same time on a large transparent substrate. After the pattern of the opaque layer has been formed at a number of locations of the transparent substrate, the substrate can be diced to form a large number of shutter elements. This further reduces the cost of fabrication of the shutter elements.